Cleaning process using a non-azeotrope forming contaminated cleaning mixture

ABSTRACT

Printed circuits are cleaned by passing them through a heated non-azeotropic solvent mixture comprising a halogenated hydrocarbon solvent preferably fluorochlorohydrocarbons containing two or three carbon atoms and an auxiliary solvent such as n-butanol, alkoxy alcohols, or cyclic ethers contained in a sump compartment and then rinsing them in a similar body of a solvent mixture but containing a lower proportion of the auxiliary solvent in a rinsing compartment.

United States Patent 1 1 1111 3,904,430 Tipping et a]. Sept. 9, 1975 [5CLEANING PROCESS USING A 3,340,199 9/1967 Clay et a1 252/171 NONAZEOTROPE FORMING 3,553,142 l/l97l Fiegiel et a1. 252/141 3,723,3323/1973 Barton 252/171 CONTAMINATED CLEANING MIXTURE 3,733,218 5/1973Begun 252/171 [75] Inventors: James William Tipping; Bernard Patrickwhim, both of Runcom FOREIGN PATENTS OR APPLICATIONS England 655,644l/l963 Canada 252/171 [73] Assignee: 'Imperial Chemical Industries OTHERPUBLICATIONS Limited, London, gland The Technology of Solvents andPlasticizers; Doolit- 2 F! d: A 24 1973 tle; pages 591-599, John Wiley &Sons, Inc.; New [2 1 York, 1954. [21] Appl. N0.: 391,258

Primary ExaminerS. Leon Bashore 30 Foreign Application priority DataAssistant Examinerl 1. Steven Alvo Sept. 7, 1972 United Kingdom 41484/72Attorney Agent or firm-Cushman Darby &

Cushman [52] US. Cl. 134/11; 134/12; 134/26;

134/30; 134/3]; 134/38; 252/171 ABSTRACT [51] Int. Cl B08b 7/04; B08b3/00 primed circuits are cleaned by passing them through a 1 Field ofSearch 134/11, 12, 15, 31, 26, heated non-azeotropic solvent mixturecomprising 11 134/30, 38;252/171 halogenated hydrocarbon solventpreferably fluorochlorohydrocarbons containing two or three carbonReferemies Clted atoms and an auxiliary solvent such as n-butanol, alk-UNITED STATES PATENTS oxy alcohols, or cyclic ethcrs contained in a sump2,104,992 1/1938 Hollerer 252/171 x Compartment and rinsing them in aSimilar y 2,153.577 4/1939 Levine 134/26 of a l n mixture ut Containinga lower proportion 2,162,656 6/1939 Warrington 134/12 of the auxiliarysolvent in a rinsing compartment. 2 220,124 11/1940 Seaton 29/D1G. 7 0 03,085,918 4/1963 Sherliker 134/30 37 Clalms, 1 D'awmg Flgure CLEANINGPROCESS USING A NON-AZEOTROPE FORMING CONTAMINATED CLEANING MIXTURE Thisinvention relates to a method of cleaning contaminated articles andapparatus therefor.

Contaminated articles, especially printed circuits, on which may bemounted components containing polymeric insulations and markings andhaving rosin-based soldering fluxes attached thereto do requirecleaning.- Such articles may be treated with halogenated hydrocarbonsolvents .especially fluorochlorohydrocarbon solvents such as forinstance 1,1,2-trichloro-l ,2,2 trifluoroethane in association withauxiliary solvents. In particular the articles may be treated withazeotropic mixtures of the solvents or with mixtures capable of formingan azeotrope. It is usual to employ azeotropic mixtures at the boil.Azeotropic mixtures of the primary solvent with an auxiliary solvent areuseful for some purposes but such mixtures may not contain sufficient ofthe auxiliary solvent to remove the fluxes effectively.Azeotrope-forming mixtures comprising a mixture of the primary solventwith a high concentration of the auxiliary solvent well above that ofthe azeotrope or mixtures of solvents not containing such an azeotropemight be expected to solve the problem and these are utilised at arelatively cold temperature, that is, near to ambient temperature, e.g.,C or less. If desired such treatment may be followed by then bringingthe article into contact with a clean cold mixture of the primarysolvent and the auxiliary solvent in a separate tank, or by thenbringing the article into contact with a liquid produced from a body ofthe liquid mixture by evaporation and condensation of the vapour thearticle being within a vapour zone and allowing the liquid to drain offthe article. In such methods there may be quite serious loss of solventdue to an adhering layer of cold solvent on articles withdrawn from theplant and- /or in that in certain applications a sufficiently highdegree of removal of flux from the contaminated article may still not beeffected.

Cleaning plants for treating contaminated articles may comprise a liquortank in which the articles are im mersed, the liquor tank being providedwith an overflow to a sump tank. This latter tank contains a relativelysmall amount of liquor which is held at the boil and the purpose of thesump is not to bring the articles into contact with the liquor but tocollect dirty solvent. Vapour above the level of the liquor is condensedand returned to the liquor tank thereby ensuring the presence of cleansolvent in the liquor tank while impure solvent overflows to the sump.The articles are withdrawn from the liquor tank then through a vapourzone and finally out of the plant. Use of hot or boiling solventmixtures containing high concentrations of auxiliary solvents of higherboiling point than the primary solvent in both tanks of such plantswould not be effective inasmuch as there is depletion of the auxiliarysolvent from the liquor tank and increase of auxiliary solvent in thesump. The system is therefore not stable and will not do the cleaningjob required of it.

We now provide a method of cleaning contaminated articles which is quitedistinct from what has been previously used, which permits effectivecleaning of con taminated articles at elevated temperatures and which isstable.

According to the present invention we provide a method of cleaning acontaminated article in a system which comprises using a solvent mixturecomprising a halogenated hydrocarbon solvent and an auxiliary solventwhich does not form an azeotrope with said halogenated hydrocarbonsolvent wherein the article is contacted with a first heated liquidmixture comprising the halogenated hydrocarbon solvent and a proportionof the auxiliary solvent which is at least 2.5 parts by weight perhundred parts by weight of the halogenated hydrocarbon solvent, and thearticle is subsequently rinsed with a second liquid mixture containing alower proportion of the auxiliary solvent than is contained in the firstliquid mixture.

Preferably there is introduced into the cleaning system, continuously orintermittently from an external source, a supply of the halogenatedhydrocarbon solvent and/or the auxiliary solvent in quantities requiredto maintain the composition of the said first and second liquid mixturessubstantially constant.

Preferably in any method of carrying out the process of the inventionthe second liquid mixture is allso heated.

It is preferred that at least part and preferably all of the condensatefrom the vapour of the first and second liquid mixtures be returned to avessel (the rinsing compartment) containing the second liquid mixture,the latter overflowing to a vessel containing the first liquid mixture.

According to a preferred method of carrying out the process of theinvention we provide a method of cleaning a contaminated article whichcomprises immersing the article in said first heated liquid mixture,said mixture being contained in a sump compartment of a cleaning system,and subsequently passing the article into said second liquid mixture ina rinsing compartment of said system, the first and second liquidmixtures having a common vapour Zone. Preferably the second liquidmixture is also heated. This second liquid mixture is in equilibriumwith the vapour in the vapour zone above the sump and rinsingcompartments and with the said mixture in the sump compartment. Aplurality of rinsing compartments may be employed if desired.

Suitably the appropriate proportion of the halogenated hydrocarbonsolvent and the auxiliary solvent is introduced continuously orintermittently into the sump and/or rinsing compartments of the cleaningsystem.

In said preferred form of carrying out the present process at leastpart, and preferably all, of the liquid obtained by condensation ofvapours from the common vapour zone is returned to the rinsingcompartment. Most suitably it is also arranged that liquid mixtureoverflows from the rinsing compartment and passes to the sumpcompartment.

Preferably the first headed liquid mixture contains at least five partsby weight of the auxiliary solvent per hundred parts by weight of thetotal mixture.

Ofthe halogenated hydrocarbons which may be used in the present processthere may be mentioned fluorochlorohydrocarbons especially thosecontaining two or three carbon atoms, for example l,l,2,2-tetrachloro-1,2-difluoroethane. l,l ,Z-trichlorol ,2,2- trifluoroethane givesespecially good results.

Examples of auxiliary solvents which can be employed in association withthe halogenated hydrocarbon include alcohols (which do not formazeotropes with the halogenated hydrocarbon solvent), for example,n-butanol; alkoxy alcohols for example 2-methoxy ethanol, Z-ethoxyethanol and Z-butoxy ethanol; and cyclic others, for example1,4-dioxanc. The nonazcotropic solvent mixtures used in the presentprocess are those which do not have the capability of forming anazeotrope as for example they do not include mixtures ofl,l,2-trichloro-l,2,2-trifluoroethane having a high concentration ofisopropyl alcohol which on fractionation give an azeotrope of saidtrichlorotrifluoroethane and a small proportion of isopropyl alcohol.

[n the sump compartment when using for example 1,1 ,2-trichlorol,2,2-trifiuoroethane the concentration of the auxiliary solvent issuitably not greater than 9071 by weight of the liquid mixture therein.It is preferred that said concentration of auxiliary solvent is notgreater than 7071 and it is particularly preferred that thisconcentration is not greater than 40% by weight of the liquid mixture inthe sump compartment.

The concentration of the auxiliary solvent in the rinsing tank isdifferent from that of the liquid mixture in the sump compartment. Theliquid mixture in the rinsing compartment, once stable conditions areestablished, has a concentration of the auxiliary solvent in the primaryhalogenated hydrocarbon solvent which generally corresponds to 0.5 to byweight of the said liquid mixture. In operating the present process,with supply from an external source of said primary and auxiliarysolvent, this second liquid mixture is in equilibrium with the firstliquid mixture in the sump compartment and with the vapour in the commonvapour zone, that is, the composition of said second liquid mixture issubstantially constant while that in the sump compartment and the vapourin the common vapour zone remain substantially constant.

The concentration of the auxiliary solvent in the halogenatedhydrocarbon solvent in the sump and rinsing compartments will alsodiffer according to the particular auxiliary solvent used. Thus whenusing 1,1,2-trichlorol ,2,2-trifluoroethanc as the primary solventuseful results can be obtained, for example, when using a concentrationof to by weight of n-butanol (auxiliary solvent) with reference to thetotal liquid mixture in the sump compartment and l to 1.5% by weightn-butanol in the rinse compartment, both liquids being at the boil. In asimilar way there may be usefully employed a concentration of 8 to 12%by weight 1,4-dioxane in the sump compartment and 2 to 4% by weightl,4-dioxane in the rinse comparting, both liquid being employed at theboil.

The external source of halogenated hydrocarbon solvent and auxiliarysolvent supplied to the system will vary according to the particularsolvents employed. The two solvents may be introduced separately intothe sump and/or rinsing compartments or a preformed mixture of thesolvents can be employed. The amount of said solvents introduced intothe system is preferably that required to maintain the level constant inthe sump compartment.

The solvent mixtures in the sump and rinsing compartments are held at atemperature greater than ambient temperature (the latter in someclimates being 20C). Usually the mixtures are held at a temperature ofat least C and preferably at least C. More preferably the solventmixtures are held at the boiling point which in the case of mixturesusing 1,1,2-trichlorol ,2,2-trifluoroethane as the primary solvent areoften in the range C to C or higher.

The procedure of immersing the contaminated article in said heatedliquid mixture (which does not form an azeotrope) in the sumpcompartment of a cleaning system alone or in association with thefurther immersb n in said heated liquid mixture in the rinsingcompartment is inter alia an unusual feature of the present process.

Other solvents or additives can be added to the solvent compositionsused in the present process if it is desired to modify their cleaning orsolvent power. Suitable additives include cationic, anionic, andnonionic detergents. Water may also be added in some circumstances,particularly when the compositions include a detergent but this is notessential.

It is usually unnecessary to use stabilisers in the solvent mixtures.However it is possible that stabilisers may be desirable under corrosiveconditions, for example those conditions in which the solvent mixturescome into contact with light metals for example zinc and aluminium.

The present process is useful in a wide range of applications forcleaning contaminated articles including removal of tenacious solderingfluxes from electrical equipment. In particular it is useful forremoving fluxes from equipment carried on a plastic or resin substratewhile not damaging the boards or components thereon.

In another method of carrying out the process of the invention acontaminated article is contacted with said first heated liquid mixturein a cleaning system having one compartment (sump compartment), thearticle is withdrawn to the vapour zone above said compartment andtherein is rinsed with said second liquid mixture which is firstobtained by condensation of vapours from said vapour zone. In amodification of that process the contaminated article after immersion insaid first heated liquid mixture is withdrawn through and out of saidvapour zone, allowed to cool above the vapour zone but within thecleaning vessel and is then returned to said vapour zone where it isrinsed by said second liquid mixture derived by direct condensation ofvapours from said vapour zone on to said cooled article.

In another variant of the present process the first and second liquidmixtures are in compartments separate from each other and not possessinga common vapour zone, condensed liquid from vapour above said firstheated mixture being returned to the second heated mixture.

One suitable type of apparatus which can be used in the present processis shown in the accompanying drawing (FIG. 1 which represents aschematic view in vertical cross section and are not to scale. In thedrawing, a vessel 1 is divided into a sump compartment 2 and a rinsecompartment 4 separated by wall 3. Sump compartment 2 contains a body ofthe first mixture (which does not form an azeotrope) of sufficient depthfor the contaminated articles to be immersed therein, and is providedwith heater 5. Rinse compartment 4 adjacent to the sump compartment 2 isprovided with heater 6 and contains a volume of the second liquidmixture as hereinbefore described, of composition different from that inthe sump compartment 2. Vapour zone 7 communicates with both sump andrinsing compartments 2 and 4. Cooling coil 8 is fitted in the vapourzone 7 to condense vapour, together with trough 9. The

latter is for collection of condensed liquor and pipe is for return ofcondensate to rinsing compartment 4. A cold line 11 is attached to theoutside of the vessel, through which a cooling medium can be passed toprovide extra cooling. An inlet 12 may be provided for feeding a supplyfrom an external source of a mixture of primary and auxiliary solventsas hereinbefore described; it is shown in the drawing as leading intothe sump compartment 2, but it can be placed so as to feed into thesystem at any convenient point. Means (not detailed) are also providedto transport the contaminated articles through the apparatus, the trackof the articles being represented by the line beginning at the entrypoint 13 and leading through sump compartment 2, vapour zone 7 andrinsing compartment 4 and ending at exit point 14.

in operation, the solvent mixtures in both compartments 2 and 4 areheated and the vapours from the mix in the vapour zone 7, condense onthe cooling coil 8 and flow back as liquid into rinsing compartment 4,from which excess liquid flows over wall 3 into the sump compartment 2.This continuing flow of liquid and vapour maintains a clean rinsingliquid in compartment 4 and ensures that the dirt and contaminationaccumulate in compartment 2, from which it may be removed in anyconvenient manner, for example by periodically removing all or part ofthe dirty liquid. The levels of liquid are maintained by adding freshsolvent mixture. as required, to make up for any amounts thus removed.

The present invention includes with its scope an apparatus whichcomprises a vessel having a sump com partment provided with a heatingdevice and a volume of a first non-azeotrope forming liquid mixture ashereinbefore described of depth sufficient to immerse the contaminatedarticles, a rinsing compartment or compartments adjacent to the sumpcompartment provided with a heating device or devices and containing avolume of liquid as hereinbefore described and being different from thatin the sump compartment, said rinsing compartment being adapted topermit overflow of liquid into the sump compartment, a vapour zonecommunicating with both sump and rinsing compartments, cooling meansadapted to permit condensation of vapour above the sump and rinsingcompartments and means adapted to return condensed liquor to the rinsingcompartment or compartments.

The following Examples illustrate but do not limit the invention.

EXAMPLE 1 The material to be cleaned consisted of printed circuit boardsconsisting of resin-bonded paper boards on which adhered a deposit offlux. The flux coating had been produced by applying one brush stroke offlux to the board, followed by 2 minutes drying at 70C, dipsoldering for5 seconds at 250C, followed by a 15- minute wait. The flux was anactivated rosin-based fluxes available commercially under the Trade MarkZeva C4,

The method of cleaning was as previously described and involved dippingthe contaminated articles in the sump compartment of a cleaning plantfor a period of one minute followed by immersion in an adjoining rinsingcompartment for ten seconds. The solvent mixture in the sump consistedof l,l,2-trichloro-I ,2,2- trifluoroethane containing 14.8% by weight ofnbutanol'. The mixture in the rinsing tank consisted of 98.8% by weightof said trichlorotrifluoroethane and 1.2% by weight of nbutanol. Bothliquids were maintained at the boil. Vapour above the liquid levels wascondensed by cooling coils and condensate was returned to the rinsingtankv A mixture of said trichlorotrifluoroethane containing 8.4% byweight n-butanol was required to maintain the composition and level ofthe liquids in the sump and rinsing compartments.

All traces of flux residues were removed by this treatment withoutdamaging the boards.

Example 2 In this Example the solvent mixtures for treating printedcircuit boards were as follows:

i. Sump: l,1,2-trichloro-l,2,2-trifluoroethane containing 93% by weightof l,4-dioxane,

ii. Rinse: 97.3% by weight of said trichlorofluoroethane and 2.7% byweight 1,4-dioxane,

iii. the mixture'required to maintain the composition and level ofliquids in the sump and rinsing compartment consisted of saidtrichlorotrifluoroethanc containing 9.8% 1,4-dioxane.

All traces of flux residues (as described in Example 1) were removed bythis treatment without damaging the boards.

What we claim is:

l. A method of cleaning a contaminated article which comprisescontacting the article with a first contaminated liquid mixture which isheated to at least 30C.. removing contamination from the article andaccumulating the removed contamination in the first liquid mixture. saidfirst liquid mixture containing a fluorinated hydrocarbon solvent and anorganic auxiliary solvent which together form a non-azeotrope-formingmixture and wherein the concentration of the auxiliary solvent in themixture is at least 2.5 parts by weight per hundred parts of thefluorinated hydrocarbon solvent, and subsequently rinsing the articlewith a second liquid non-azeotrope-forming mixture containing thefluorinated hydrocarbon solvent and a proportion of the auxiliarysolvent which is lower than that contained in the first liquid mixture,but at least equal to 0.5% by weight of said second liquid mixture.

2. A method as claimed in claim 1 in which there is introduced,continuously or intermittently from an external source a supply of thefluorinated hydrocarbon solvent or the auxiliary solvent in quantitiesrequired to maintain the composition of the said first and second liquidmixtures substantially constant.

3. A method as claimed in claim 2 in which the external source of thefluorinated hydrocarbon solvent and the auxiliary solvent are introducedinto a sump compartment.

4. A method as claimed in claim 1 in which the sec ond liquid mixture isalso heated.

5. A method as claimed in claim 1 which comprises immersing the articlein said first heated liquid mixture, said mixture being contained in asump compartment and subsequently passing the article into said secondliquid mixture in a rinsing compartment, the first and second liquidmixtures having a common vapour zone.

6. A method as claimed in claim 5 in which at least part of thecondensate from the vapour of the first and second liquid mixtures isreturned to the rinsing compartment containing the second liquidmixture.

7. A method as claimed in claim in which liquid mixture overflows fromsaid rinsing compartment to said sump compartment.

8. A method as claimed in claim 5 in which there is employed a pluralityof rinsing compartments.

9. A method according to claim 5 in which the fluori nated hydrocarbonis l. l ,2trichloro-l .22- trifluoroethane.

10. A method as claimed in claim 5 in which the auxiliary solvent isn-butanol.

11. A method as claimed in claim 5 in which the auxiliary solvent is analkoxy alcohol.

12. A method as claimed in claim 11 in which the alkoxy alcohol is2-ethoxy ethanol.

13. A method claimed in claim 1 in which the first liquid mixturecontains at least five parts by weight of the auxiliary solvent perhundred parts by weight of the total mixture.

14. A method as claimed in claim 13 in which the fluorinated hydrocarbonsolvent is a fluorochlorohydrocarbon.

15. A method as claimed in claim 14 in which the fluorochlorohydrocarboncontains two or three carbon atoms.

16. A method as claimed in claim 15 in which the fluorochlorohydrocarbonis l,l,2-trichloro-1.2,2- trifluoroethane.

17. A method as claimed in claim 1 is which the auxiliary solvent isn-butanol.

18. A method as claimed in claim 1 in which the auxiliary solvent is analkoxy alcohol.

19. A method as claimed in claim 18 in which the alkoxy alcohol isZ-methoxy ethanol.

20. A method as claimed in claim 18 in which the alkoxy alcohol isZ-ethoxy ethanol.

21. A method as claimed in claim 18 in which the alkoxy alcohol isZ-butoxy ethanol.

22. A method as claimed in claim 1 in which the auxiliary solvent is acyclic ether.

23. A method as claimed in claim 22 in which the cyclic ether is l4-dioxane.

24. A method as claimed in claim 3 in which the concentration oftheauxiliary solvent in the sump compartment is not greater than 90% byweight of the liquid mixture therein.

25. A method as claimed in claim 24 in which the concentration ofauxiliary solvent in the sump compartment is not greater than 70?! ofthe liquid mixture therein.

26. A method as claimed in claim 25 in which the concentration ofauxiliary solvent in the sump compartment is not greater than 4071 byweight of the liquid mixture therein.

27. A method as claimed in claim 1 in which the concentration ofauxiliary solvent in the second liquid mixture is in the range of 0.5 to10% by weight of the sec- 0nd liquid mixture.

28. A method as claimed in claim 1 in which the auxiliary solvent isn-butanol and the fluorinated hydrocarbon solvent is 1.1 ,Z-trichloro-l,2,2-trifluoroethane and the concentration of n-butanol in 1,1,2-trichloro-l ,2,2- trifluoroethane employed in the first liquidmixture in the sump compartment is l5to 25% by weight with reference tothe total liquid mixture.

29. A method as claimed in claim 28 in which the concentration ofn-butanol in said trichlorotrifluoroethane employed in the second liquidmixture in the rinsing compartment is l to 1.5% by weight with referenceto the total liquid mixture.

30. A method as claimed in claim 1 in which the auxiliary solvent is1,4-dioxane and the fluorinated hydrocarbon solvent is1,1,2-trichloro-l,2,2-trifluoroethane and the concentration of1,4-dioxane in said trichlorotrifluoroethane employed in the firstliquid mixture in the sump compartment is 8 to 12% by weight withreference to the total liquid mixture.

31. A method as claimed in claim 30 in which the concentration of1,4-dioxane in said triehlorotrifluoroethane employed in the secondliquid mixture in the rinsing compartment is 2 to 4% by weight withreference to the total liquid mixture.

32. A method as claimed in claim 1 wherein the temperature of the firstand second liquid mixtures is at least 30C and up to the boiling pointof the mixtures.

33. A method as claimed in claim 32 wherein the temperature of saidmixtures is at least C and up to the boiling point of the mixtures.

34. A method as claimed in claim 33 wherein the liquid mixtures have1,1,2-trichloro-l,2.2-trifiuoroethane as the fluorinated hydrocarbonsolvent and the temperature of such mixtures is in the range C to C.

35. A method as claimed in claim 1 in which the contaminated article iscontacted with said first liquid mixturc. withdrawn to a vapour zoneabove said first liquid mixture and rinsed with said second liquidmixture which is obtained by condensation of vapors from the vapor zone.

36. A method as claimed in claim 35 in which the article is immersed insaid first liquid mixture, withdrawn through and out of said vapor zone,allowed to cool above the vapour zone but within a cleaning vesselcontaining the liquid mixtures and then returned to said vapour zonewhere it is rinsed by said second liquid mixture derived by condensationof vapours from said vapour zone on to said cooled article.

37. A method as claimed in claim 1 wherein said first and second liquidmixtures are in compartments separate from each other and not possessinga common vapor zone. and vapours from above said first liquid mixtureare condensed and returned to the second liq-

1. A METHOD OF CLEANING A CONTAMINATED ARTICLE WHICH COMPRISESCONTACTING THE ARTICLE WITH A FIRST CONTAMINATED LIQUID MIXTURE WHICH ISHEATED TO AT LEAST 30*C., REMOVING CONTAMINATION FROM THE ARTICLE ANDACCUMULATING THE REMOVED CONTAMINATION IN THE FIRST LIQUID MIXTURE, SAIDFIRST LIQUID MIXTURE CONTAINING A FLUORINATED HYDROCARBON SOLVENT AND ANORGANIC AUXILIARY SOLVENT WHICH TOGETHER FORM A NON-AZEOTROPE-FORMINGMIXTURE AND WHEREIN THE CONCENTRATION OF THE AUXILIARY SOLVET IN THEMIXTURE IS AT LEAST 2.5 PARTS BY WEIGHT PER HUNDRED PARTS OF THEFLUORINAED HYDROCARBON SOLVENT, AND SUBSEQUENTLY RINSING THE ARTICLEWITH A SECOND LIQUID NON-AZEOTROPEFORMING MIXTURE CONTAINING THEFLUORINATED HYDROCARBON SOLVENT AND A PROPORITION OF THE AUXILIARYSOLVENT WHICH IS LOWER THAN THAT CONTAINED IN THE FIRST LIQID MIXTURE,BUT AT LEAST EQUAL TO 0.5% BY WEIGHT OF SAID SECOND LIQUID MIXTURE.
 2. Amethod as claimed in claim 1 in which there is introduced, continuouslyor intermittently from an external source a supply of the fluorinatedhydrocarbon solvent or the auxiliary solvent in quantities required tomaintain the composition of the said first and second liquid mixturessubstantially constant.
 3. A method as claimed in claim 2 in which theexternal source of the fluorinated hydrocarbon solvent and the auxiliarysolvent are introduced into a sump compartment.
 4. A method as claimedin claim 1 in which the second liquid mixture is also heated.
 5. Amethod as claimed in claim 1 which comprises immersing the article insaid first heated liquid mixture, said mixture being contained in a sumpcompartment and subsequently passing the article into said second liquidmixture in a rinsing compartment, the first and second liquid mixtureshaving a common vapour zone.
 6. A method as claimed in claim 5 in whichat least part of the condensate from the vapour of the first and secondliquid mixtures is returned to the rinsing compartment containing thesecond liquid mixture.
 7. A method as claimed in claim 5 in which liquidmixture overflows from said rinsing compartment to said sumpcompartment.
 8. A method as claimed in claim 5 in which there isemployed a plurality of rinsing compartments.
 9. A method according toclaim 5 in which the fluorinated hydrocarbon is1,1,2-trichloro-1,2,2-trifluoroethane.
 10. A method as claimed in claim5 in which the auxiliary solvent is n-butanol.
 11. A method as claimedin claim 5 in which the auxiliary solvent is an alkoxy alcohol.
 12. Amethod as claimed in claim 11 in which the alkoxy alcohol is 2-ethoxyethanol.
 13. A method as claimed in claim 1 in which the first liquidmixture contains at least five parts by weight of the auxiliary solventper hundred parts by weight of the total mixture.
 14. A method asclaimed in claim 13 in which the fluorinated hydrocarbon solvent is afluorochlorohydrocarbon.
 15. A method as claimed in claim 14 in whichthe fluorochlorohydrocarbon contains two or three carbon atoms.
 16. Amethod as claimed in claim 15 in which the fluorochlorohydrocarbon is1,1,2-trichloro-1,2,2-trifluoroethane.
 17. A method as claimed in claim1 is which the auxiliary solvent is n-butanol.
 18. A method as claimedin claim 1 in which the auxiliary solvent is an alkoxy alcohol.
 19. Amethod as claimed in claim 18 in which the alkoxy alcohol is 2-methoxyethanol.
 20. A method as claimed in claim 18 in which the alkoxy alcoholis 2-ethoxy ethanol.
 21. A method as claimed in claim 18 in which thealkoxy alcohol is 2-butoxy ethanol.
 22. A method as claimed in claim 1in which the auxiliary solvent is a cyclic ether.
 23. A method asclaimed in claim 22 in which the cyclic ether is 1,4-dioxane.
 24. Amethod as claimed in claim 3 in which the concentration of the auxiliarysolvent in the sump compartment is not greater than 90% by weight of theliquid mixture therein.
 25. A method as claimed in claim 24 in which theconcentration of auxiliary solvent in the sump comparTment is notgreater than 70% of the liquid mixture therein.
 26. A method as claimedin claim 25 in which the concentration of auxiliary solvent in the sumpcompartment is not greater than 40% by weight of the liquid mixturetherein.
 27. A method as claimed in claim 1 in which the concentrationof auxiliary solvent in the second liquid mixture is in the range of 0.5to 10% by weight of the second liquid mixture.
 28. A method as claimedin claim 1 in which the auxiliary solvent is n-butanol and thefluorinated hydrocarbon solvent is 1,1,2-trichloro-1,2,2-trifluoroethaneand the concentration of n-butanol in1,1,2-trichloro-1,2,2-trifluoroethane employed in the first liquidmixture in the sump compartment is 15to 25% by weight with reference tothe total liquid mixture.
 29. A method as claimed in claim 28 in whichthe concentration of n-butanol in said trichlorotrifluoroethane employedin the second liquid mixture in the rinsing compartment is 1 to 1.5% byweight with reference to the total liquid mixture.
 30. A method asclaimed in claim 1 in which the auxiliary solvent is 1,4-dioxane and thefluorinated hydrocarbon solvent is 1,1,2-trichloro-1,2,2-trifluoroethaneand the concentration of 1, 4-dioxane in said trichlorotrifluoroethaneemployed in the first liquid mixture in the sump compartment is 8 to 12%by weight with reference to the total liquid mixture.
 31. A method asclaimed in claim 30 in which the concentration of 1,4-dioxane in saidtrichlorotrifluoroethane employed in the second liquid mixture in therinsing compartment is 2 to 4% by weight with reference to the totalliquid mixture.
 32. A method as claimed in claim 1 wherein thetemperature of the first and second liquid mixtures is at least 30*C andup to the boiling point of the mixtures.
 33. A method as claimed inclaim 32 wherein the temperature of said mixtures is at least 40*C andup to the boiling point of the mixtures.
 34. A method as claimed inclaim 33 wherein the liquid mixtures have1,1,2-trichloro-1,2,2-trifluoroethane as the fluorinated hydrocarbonsolvent and the temperature of such mixtures is in the range 45*C to50*C.
 35. A method as claimed in claim 1 in which the contaminatedarticle is contacted with said first liquid mixture, withdrawn to avapour zone above said first liquid mixture and rinsed with said secondliquid mixture which is obtained by condensation of vapors from thevapor zone.
 36. A method as claimed in claim 35 in which the article isimmersed in said first liquid mixture, withdrawn through and out of saidvapor zone, allowed to cool above the vapour zone but within a cleaningvessel containing the liquid mixtures and then returned to said vapourzone where it is rinsed by said second liquid mixture derived bycondensation of vapours from said vapour zone on to said cooled article.37. A method as claimed in claim 1 wherein said first and second liquidmixtures are in compartments separate from each other and not possessinga common vapor zone, and vapours from above said first liquid mixtureare condensed and returned to the second liquid mixture.